The bone substitutes are mainly used for refilling and recovering materials to repair the cavities of bone defect caused by accidents or bone diseases. Clinically, bone cement is now mostly used for the filling material. The major content of bone cement is polymethymethacrylate (PMMA), which may release its unit, which may interfere with the human-body functions, after implanted into human body. For examples, it may cause uncomfortable feeling due to the decrease of blood pressure, and local temperature increase up to 60.degree. C., due to the polymeric reaction, may cause the death of bone cells. On the other hand, the biocompatibility of PMMA is not very good. It will not form a good linkage with ambient bone tissue after implant, and it will not be absorbed or replaced by the tissue to improve the formation of new bones. Consequently, bone cement is not perfect to be used as a bone substitute.
Besides, bioceramics like hydroxyapatite (HAP) and tricalcium phosphate (.beta.-TCP) in dense or block are directly filled into bone defect to repair the bone. Though the biocompatibility is good and part of implant materials can be absorbed and replaced by natural bones, the practical implant effect is limited. This is because when the implant powders are filled in, body liquid or blood may often flush the implant materials away from the wound, which may affect the practical implant effect. Even though the implant powders could be made into various porous blocks by molds of various shapes and sizes; however, there could be an important problem occurred during the clinical application. That is: the area and dimension of the bone defect location cannot be exactly same with those we predicted. When this happens, it is necessary to either enlarge the wound or trim the implant block to match each other. This will prolong the operation period and hurt the wound again. Furthermore, there may exit interstice between the wound and implant blocks, which may be obstructive for the cure of wounds.
Consequently, the major direction of development in the implant materials for bone defect is to investigate the composite materials, which are composed of either degradable and synthetic polymers, or mixture of natural polymers and ceramic powders. For example, the composite material composed of Apatite-wollastonite containing glass ceramic-fibrin mixtures was proposed by Yamamuro et al. (1988). Another example is the composite material composed of tricalcium phosphate-gelatin mixtures by Lin et al. (1993). Both materials were studied to become the bone implant substitutes, and both showed good biocompatibility. Furthermore, components for improving bone cell growth, like DMB, BMP and so on, are able to be mixed with these two materials easily during the synthesis process. This is why using above composites as bone implant substitutes became a popular direction of recent studies in this field.